The present invention is related to a liquid crystal cell, more particularly to a liquid crystal cell which is including one substrate coated with rubbed alignment layer and the other substrate coated with photo-aligned alignment layer, and the method for fabricating that.
Generally, the liquid crystal cell is comprising of two substrates and liquid crystal formed between these substrates, the liquid crystal comprising anisotropic molecules. To provide an orderly alignment of liquid crystal in the cell for the uniform brightness and the high contrast ratio of the liquid crystal cell, convnetionally rubbing is carried out on alignment layers coating substrate. The rubbing is mechanical firiction on the alignment layer so as to provide a pretilt of liquid crystal molecules defined by a pretilt angle and a pretilt angle direction. The pretilt angle refers a polar angle and the pretilt angle direction refers a azimuthal angle between the surface of alignment layer and the pretilt.
The pretilt of a liquid crystal molecule adjacent a first alignment layer is called a first pretilt of a first alignment layer, and the pretilt of a liquid crystal molecule adjacent a second alignment layer is called a second pretilt of a second alignment layer. Thereby, the pretilt of a liquid crystal molecule in the midle of two layers is determed by the interaction between pretilts of the first and second alignment layer.
The liquid crystal cell is classified a vertical aligned liquid crystal cell and a horizontal aligned liquid crystal cell depending the pretilt angle. The vertical aligned liquid crystal cell typically defines a liquid crystal cell having a pretilt angle of an alignment layer larger than 60xc2x0, the horizontal aligned liquid crystal cell typically refers a liquid crystal cell having a pretilt angle of an alignment layer less than 5xc2x0.
There are several modes liquid crystal cell according to relationships between a first pretilt angle direction of a first alignment layer and a second pretilt angle direction of a second alignment layer facing the first substrate. If the first pretilt angle direction is perpendicular to the second alignment direction, it is called a twisted nematic(TN) mode liquid crystal cell. If they are parallel with each other, the liquid crystal cell is called an electrically controlled birefringence(ECB) mode liquid crystal cell and a bend mode liquid crystal cell. In addition, it is called a In-Plane Switching(IPS) mode liquid crystal cell if a pretilt angle direction is shift depending on the voltafe.
A conventionally used liquid crystal display is mainly a twisted nematic liquid crystal display (TNLCD), in which the transmittance is dependent according to the viewing angle at each gray level. Especially, while the transmittance is symmetrical in the horizontal direction, the transmittance is asymmetrical in the vertical direction. Therefore, in the vertical direction, the range with inverted image phenomenon is occurred so that the vertical viewing angle becomes very limited.
To overcome said problems, a multi-domain TNLC cell such as a two-domain liquid crystal cell, and a four-domain liquid crystal cell is introduced. The multi-domain liquid crystal cell has a wider viewing angle by providing more than domains in each pixel, domains having different pretilts each other, so as to compensate the viewing angle dependence of each domain.
The most popular process to obtain said multi-domain liquid crystal cell is mechanical rubbing process, as shown in FIG. 1. Rubbing is performed mechanically on entire substrate 1 coated with alignment layer 8 such as polyimide, so that microgrooves are formed on the surface of the alignment layer 8, as shown in FIG. 1a, and FIG. 1b. To divide two domains in a pixel, in FIG. 1c and FIG. 1d, a photoresist 11 is coated entire alignment layer 8 surface, and the photoresist 11 of one domain is removed by exposing light, reverse rubbing process is carried out on one domain as shown in FIG. 1e. The remained photoresist is removed by exposing light, then, two domains are provided on the substrate 1 as shown in FIG. 1f. In the two-domain liquid crystal cell obtained thereby, the inversion of viewing angle is compensated by aforementioned process.
However, the rubbing process causes a dust particle and/or an electrostatic discharge, so the yield is reduced and/or the substrate is damaged. The manufacturing process becomes too complicated to apply industry, because the process includes a photolithography which is coating photoresist layer and removing a part of the layer by exposing light, for dividing domains.
Therefore, it is a photo-alignment method that is introduced to simplify alignment process as well as to prevent the damage of substrate. The photo-alignment is the process in which a pretilt angle direction of alignment layer is given by the irradiation of linearly polarized ultraviolet light. The alignment layer used in the photo-alignment method is mainly including PVCN (polyvinyl cinnamate). When ultraviolet light is irradiated into the photo-aligned layer coating the substrate, it causes cyclo-addition between the cinnamoyl groups of cinnamic acid side chains that belong to different photopolymers. Thereby, the direction of the photopolymer configuration i.e., the pretilt of alignment layer is aligned uniformly.
One example of the photo-alignment method is disclosured as a following process. The photo-alignment method is comprising double exposure of linearly polarized ultraviolet light into a substrate coated with PVCN to determine a pretilt, the pretilt including an alignment direction, a pretilt angle direction and pretilt angle. First linearly polarized ultraviolet light is perpendicularly irradiated into the alignment layer coating substrate so as to determine a plurality of pretilt angle direction. Then, second linearly polarized light is obliquely irradiated into the alignment layer again, to determine a pretilt angle and a pretilt angle direction. The pretilt angle and pretilt angle direction are obtained by controlling the second oblique direction relative to the substrate coated with the alignment layer.
However, the photo-alignment method has problems that the process is complicated due to the double exposure and the pretilt angle is too small, for example, the obtained pretilt angles being approximately 0.15xc2x0, 0.26xc2x0 and 0.30xc2x0 respectively when the oblique irradiation angles are 30xc2x0, 45xc2x0 and 60xc2x0. In addition, it takes long time to irradiate light into the alignment layer so total tact time is prolonged, as well, the alignment stability of photo-alignment method is weaker than that of rubbing method.
An object of the present invention is to provide a liquid crystal cell having an alignment stability and a wilder viewing angle by simply process and the fabrication method thereof.
To achieve the object mentioned above, the method for fabricating the liquid crystal cell of the present invention is comprising the following steps of: providing a first alignment layer on a first substrate; rubbing said first alignment layer such that said first alignment layer has a first pretilt angle associated therewith; providing a second alignment layer on a second substrate; exposing said second alignment layer to light such that said second alignment layer has at least one second pretilt angle associated therewith; and providing a liquid crystal material between said first and second substrates.
The first alignment layer is comprising polyimide, but the second alignment layer is comprising photopolymers, the photo-polymers including polysiloxane based materials. The pretilt angle of this invention is controlled depending upon the photo-energy of the ultraviolet light irradiating.
To determined the second pretilt, this invention includes double irradiation of this invention. The double irradiation is comprising steps of: irradiating polarized light in the perpendicular direction to the second alignment layer and irradiating unpolarized light in the oblique direction to the second alignment layer. In this steps, the latter step can be prior to the former.
Another method for determining the second pretilt in this invention is using flowing effect. The method is comprising the steps of: irradiating polarized light in the perpendicular direction to the second substrate so as to determine a second pretilt angle and two pretilt angle direction; and injecting liquid crystal materials between said first substrate and said second substrate in the perpendicular direction to the polarized direction of the light so as to select a pretilt angle and a second pretilt angle direction.
These and other objects and advantages of the present invention will become clear from the following description of preferred embodiments.